PROJECT SUMMARY Accumulating data indicate that environmental influences during prenatal and early postnatal development can affect individual susceptibility to obesity throughout life. The long-term goal of this line of research is to define molecular events that mediate environmental influences on development of neuronal circuitry involved in the regulation of mammalian energy homeostasis. The hypothalamus is a likely site for coupling early environmental signals to lifelong control of body weight, and neuronal projections from the arcuate (ARH) to the paraventricular (PVH) nucleus of the hypothalamus are essential in regulating energy balance. The adipocyte-derived hormone leptin directs formation of neuronal projections from the ARH to the PVH, and distinct aspects of body weight regulation are dependent on these developmental actions of leptin, specifically during a postnatal critical period. The mechanisms that restrict formation of neuronal pathways to discrete developmental periods are largely unknown. But DNA methylation, which appears to function as a major epigenetic mark stably modulating expression of genes that control development in a variety of neural systems, is an excellent candidate. The overall hypotheses of the proposed research are that 1) the critical developmental window during which ARH neurons are responsive to the neurotrophic actions of leptin is dictated by epigenetic mechanisms, and 2) cell type-specific methylation of genic and other regulatory regions is essential for accurate targeting of functionally distinct classes of neurons, with consequences for normal metabolic physiology. We will test these hypotheses by mapping cell type-specific epigenetic expression modules in the ARH during and after closure of this critical developmental window (Aim 1). In addition, we will identify leptin-mediated epigenetic changes within subpopulations of ARH neurons that display leptin- dependent patterns of projections to the PVH (Aim 2). We will also use transgenic mice with targeted disruption of the DNA methylation machinery to determine if DNA methylation is required for closure of the critical developmental period for the neurotrophic action of leptin on ARH projections and associated metabolic physiology (Aim 3). The proposed research will objectively and definitively assess whether epigenetic mechanisms specify critical periods of brain development when leptin and perhaps other environmental factors impact brain architecture and the regulation of energy balance throughout life.